The accuracy of today's satellite laser ranging system is limited to a few cm. A significant part of this range error is due to the limitations of the atmospheric correction model. A dual color ranging experiment has been designed to investigate this source of error. When ranging to satellites at the fundamental and second harmonic frequency of a Nd:YAG laser, two different pulse round trip times are obtained simultaneously. The infrared pulse is detected by an avalanche photodiode, operated in the `Geiger mode', while the green pulse is recorded by a microchannel plate photomultiplier. For a given satellite pass, the jitter in recording the time of flight of the pulse is too high to calculate an atmospheric correction from individual measurements. Due to the many shots per satellite pass, the scatter can be significantly reduced by applying a nonlinear least squares fitting procedure to the data. The results of a large number of satellite passes are compared with the predictions of the Marini-Murray model.

In today's laser ranging technology the influence of the atmosphere belongs to the dominating contributions to the error budget of measurements. In this paper a detection technique for atmospheric dispersion measurement is described. The measurements are carried out using the Wettzell Laser Ranging System, which is designed for satellite--and lunar laser ranging. The dispersion is determined by simultaneous ranging, using laser pulses of the fundamental and second harmonic frequency of a Nd:YAG laser. While propagating through the atmosphere, the frequency dependent refractive index of atmospheric gases causes a different path delay to laser pulses of both wavelengths. As the effect is very weak and the jitter of semiconductor devices is quite high, the calculated atmospheric corrections from the data, obtained with conventional techniques, need independent proof. So a streak camera with high temporal resolution with respect to the differential path delay between both echo pulses is adopted for this purpose. Dispersion measurements to a local ground target (2.4 km of optical path) have been carried out. The received signal was observed as a double peak, one corresponding to the infrared, the other to the green laser pulse. In these experiments a RMS of 10 ps for the path delay between the received pulses was obtained.

ATLID (ATmospheric LIDar) is the ESA backscatter lidar instrument, prime candidate to be flown on a future European Earth observation mission. It will provide information on features of the Earth's atmosphere, such as top height of all cloud types and Planetary Boundary Layer aerosols, thin cloud extent, optical depth and depolarization. Based on the results of a pre- phase-A and two subsequent parallel phase-A studies, ESA decided in 1991 to initiate the ATLID Instrument Technology Predevelopment Program. It is broken into two stages: The first stage is devoted to concept selection, instrument design and breadboarding of critical technologies. The second stage will cover the design, development, assembly and testing of an advanced ATLID demonstration model. The first stage is further divided into a Phase 1, concept selection and preliminary design, which has been finished end of 1993, and a Phase 2, currently in progress, comprising the breadboarding of critical technologies and a final instrument design update. The selected instrument architecture is based on a one-axis scanning 60 cm telescope and a pulsed diode-pumped Nd:YAG laser transmitter. The estimated instrument volume, mass and power are 1.4 m X 1.6 m X 1.1 m, 200 kg and 450 W, respectively.

ATLID is a spaceborne backscatter LIDAR using a solid-state Nd-YAG laser (1.06 micrometers wavelength) and a 0.6 m diameter telescope. It is intended to fly on-board a polar platform satellite. The selected concept consists in a lightweight scanning telescope associated to a contra-rotative flywheel. A linear scanning (+/- 23 degree(s)) is used in order to achieve the required swathwidth (700 Km). The detector is a silicon Avalanche Photodiode. The instrument has been compacted to a similar volume as for currently developed ENVISAT-1 instruments. The thermal control is designed to be independent of the neighbor instruments, thus allowing ATLID to be mounted on a multi-instrument payload. A breadboarding program has been initiated for critical parts of the instrument. This paper describes the overall instrument architecture, as well as first breadboard results.

Atomic line filters take advantage of the sharp spectral features offered by electronic transitions in free atoms. The basic idea is to convert a narrow atomic absorption profile into an equally narrow optical transmission filter by detecting the fluorescence radiation that is emitted after a photon has been absorbed. We developed the metastable thallium atomic line filter from the basic physical idea to a laboratory prototype in order to evaluate the usefulness of the technology for satellite borne backscatter LIDARs. Atomic thallium provides for an unsurpassed combination of features for this purpose. The input wavelength of the filter is 535 nm, which matches the frequency doubled Nd:BEL laser. The filter is an active device, it upconverts the 535 nm input into 378 nm output that is detected by a PMT. The experimental results were as expected concerning the characteristics directly related to atomic properties, i.e. the filter has only 10 ns response time, its optical bandwidth is 0.004 nm, and its acceptance angle is only limited by the device geometry. An optimized setup, with the size constraints of a space-borne system, displays a total quantum efficiency of 2%, i.e. from input photons to detected output photons. This constitutes a remarkable and unsurpassed value for atomic line filters but is too low for the application in mind. (A ground-based, scaled-up version of our prototype would reach about 10% quantum efficiency). In addition, the 500 degree(s)C operating temperature of the vapor cell requires sophisticated thermal layout and thermal shielding, which means a lot of added mass and volume. In summary, we found that atomic line filters can be built that offer characteristics not found with other technologies, but their applicability for space borne systems is questionable.

For ATLID, a filter featuring a narrow transmission band of less than 0.2 nm and a high transmission of greater
than 50% is required. The Fabry-Perot etalon is a possible variant to achieve these specifications. The present paper
will analyse how the required specifications are met by means of a birefringent filter.

Both meteorological and climatological applications require the knowledge of water vapor and temperature profile on the global scale. At present monitoring strategies combine satellite data with ground based instrumentation in order to check up the retrievals obtained starting from satellite data measurements. Lidar systems are very important tools for this comparison, providing range resolved measurements with high vertical and temporal resolution. Two lidar systems are presently running at Napoli and Potenza (Italy, 40 degree(s)50'N - 14 degree(s)10'E and 40 degree(s)36'N - 15 degree(s)44'E). These systems are capable of providing vertical profiles of water vapor and temperature. The water vapor measurement is based on the Raman scattering technique, while the temperature measurement is based on the combined Rayleigh- Raman technique. Simultaneous measurements of the water vapor content through the application of both the Raman and D.I.A.L. techniques have been recently performed in Potenza. The simultaneous application of the two techniques reduces the uncertainty affecting the water vapor measurement, resulting in a more accurate reference for water vapor satellite measurements. Measurements carried out in both sites are shown and discussed.

Environmental sensing and atmospheric monitoring are two of the main areas where LIDAR can be
used to advance our understanding of global climate changes and the effects of industrial pollution.
The spectrum from the UV to the far-IR are used to sense a variety of atoms and molecules. However
most of the pollutants are hydrocarbons that have their fingerprint in the region of 8 to 12 rim. This
wavelength region can be partially covered by C02 lasers. Besides, their wavelength can be extended
by harmonic generation to cover the 4.6 to 5.4tm range1. Other C02 laser technology applications
are wind-measurements and combustion dynamics . The required short pulses can be generated with
intracavity and extra-cavity modulators. An attractive approach to achieving a short pulse tunable
mid-IR light source utilizes a low power tunable CW laser combined with an external widebandwidth
modulator. This paper describes the underlying physical principles of a high speed C02
modulator based on the plasma effect and the design towards low electrical power dissipation.

The Lidar In-space Technology Experiment (LITE) is a multi-wavelength backscatter lidar developed by NASA Langley Research Center to fly on the Space Shuttle. The LITE instrument is built around a three-wavelength Nd:YAG laser and a 1-meter diameter telescope. The laser operates at 10 Hz and produces about 500 mJ per pulse at 1064 nm and 532 nm, and 150 mJ per pulse at 355 nm. The objective of the LITE program is to develop the engineering processes required for space lidar and to demonstrate applications of space-based lidar to remote sensing of the atmosphere. The LITE instrument was designed to study a wide range of cloud and aerosol phenomena. To this end, a comprehensive program of scientific investigations has been planned for the upcoming mission. Simulations of on-orbit performance show the instrument has sufficient sensitivity to detect even thin cirrus on a single-shot basis. Signal averaging provides the capability of measuring the height and structure of the planetary boundary layer, aerosols in the free troposphere, the stratospheric aerosol layer, and density profiles to an altitude of 40 km. The instrument has successfully completed a ground-test phase and is scheduled to fly on the Space Shuttle Discovery for a 9- day mission in September 1994.

The importance of clouds to the earth's climate and to the prediction of climate change is now well recognized. The Lidar In-space Technology Experiment (LITE) gives our first global view of the 2D structure of clouds. Techniques that have been developed previously in the Experimental Cloud Lidar Pilot Study, and in many other groundbased applications, will be used to retrieve cloud height, depth, optical depth and extinction coefficient of clouds observed along the LITE orbits. The cloud observation from LITE are extended by the use of variable apertures at the LITE telescope focus, allowing us to study the amplitudes of laser radiation that has been multiply scattered in the clouds. Monte Carlo simulations predict that multiple scattering can cause an appreciable enhancement to the penetration of laser pulses into dense clouds, and that these effects are magnified at space shuttle ranges compared to groundbased observations. The relative magnitudes of the solar radiation reflected by the observed clouds will also be available during daytime portions of the orbits, and will be used to assist in retrieval of cloud optical properties. Some preliminary data on clouds from the LITE are illustrated and discussed.

Lidar in space have been featuring as a high sensitive active sensor for global observations of aerosol, cloud, water vapor, wind vector etc. Numerous efforts have been carried out toward realization of those by many researchers. NASDA also started a space lidar program from 1990. A present status of this program is a Phase A. The Phase A was highly directed to resolve system parameters of bookstore lidars, DIAL for measurements of global aerosol density, cloud height, vertical and horizontal distribution of water vapor concentration etc. and to develop an airborne lidar system, including high power diode-pumped Nd:YAG, Nd:YLF lasers, to obtain data for simulation of space lidar. This paper will describe the airborne lidar system with diode-pumped Nd:YAG, Nd:YLF lasers under development.

The capabilities of orbital lidars to sense cloudiness are analyzed by the example of interpretation of the signals obtained with geodesic laser altimeters. A method is described of reconstructing the optical characteristics of cloudiness from the measured duration of signal returns at predetermined levels. The probabilities of occurrence of various magnitudes of attenuation coefficient and lidar ratio obtained by us are compared with the published data. Results of analysis of cloud returns of the altimeter confirm the capability of space-based lidars to yield reliable information and allow us to provide a basis for guiding their development.

Results of laser cloud top measurements taken from space in 1982 (called PANTHER) are presented. Three sequences of land, water, and cloud data are selected. A comparison with airborne lidar data shows similarities. Using the single scattering lidar equation for these spaceborne lidar measurements one can misinterpret the data if one doesn't correct for multiple scattering.

In this paper we describe the first Russian spaceborne single frequency aerosol lidar. This lidar was especially designed for studying the cloud formations and the Earth's surface from space and it is planned now that it will be installed onboard the `Spektr' modulus of the orbiting Russian station `Mir'. The construction of the lidar consists of a transmitter-receiver, electronic system of data acquisition and transmission, and the control keyboard. This lidar has undergone the full cycle of ground tests as a part of the whole modulus `Spektr' and now it is ready for launch.

This paper presents a discussion of some methodological aspects of interpreting data that could be obtained with a spaceborne single frequency lidar. The case is considered when the lidar return signals are recorded in analog regime. Some possibilities of using such data in combination with the data obtained using passive techniques of sounding the atmosphere are also discussed in this paper. We also present in this paper some results of a closed numerical experiment on sensing stratiformis and some types of the underlying surface with a lidar from space. Interpretation of data obtained with a spaceborne range finder is also discussed.

Cirrus clouds are a specific atmospheric formation that essentially influence on the radiation balance in the atmosphere, hamper the operation of the Earth-Space optical communication systems, and distort the results of instrumental observations of the Earth from space. Very often cirrus clouds are invisible for spaceborne instruments which makes it difficult to properly account for the distortions they introduce into the observations. These and other circumstances make the detection of cirrus clouds and measurement of the characteristics with a spaceborne lidar an urgent problem of lidar technology. Sounding of cirrus clouds with a lidar has some peculiarities compared to sounding of lower level clouds, first of all because these clouds are mainly composed of crystal particles. Orientation of such particles that can occur due to the action of gravity, aerodynamic, and electrostatic forces makes lidar return signals strongly dependent on the angle at which sounding radiation is incident on the particles and on the state of sounding radiation polarization. To illustrate this statement we remind the existence of the effect of anomalous backscattering discussed. The effect occurs due to specular reflection of light from the plane surfaces of ice crystals.

The European Space Agency is supporting the development of key technologies for future spaceborne Doppler wind lidar instruments. The focus for this work is the ALADIN (Atmospheric Laser Doppler Instrument) program which is directed towards the establishment of a coherent CO2-laser based lidar system to improve operational meteorology and climate science. Technology support for this program has to date centered on the development of a 10 J class electron-beam sustained CO2 laser. The stringent alignment tolerances necessary for coherent spaceborne lidar systems have been addressed by the manufacture and test of a single-axis lag-angle and image-motion compensator. Receiver related work has to date concentrated on advanced signal processing algorithms. An alternative approach to coherent DWL (Doppler Wind Lidar) that shows promise for the longer term is a 2 micrometers lidar based upon InGaAs detection and the all-solid-state Tm:Ho:Host laser. These technologies, which may also find application within a spaceborne water-vapor DIAL (Differential Absorption Lidar), have been the subject of experimental activities in relation to spectroscopy, cw lasing and heterodyne efficiency. Also under development is incoherent DWL operating in the ultra-violet which, whilst not capable of the ultimate sensitivity of coherent lidar, is attractive for its reduced alignment tolerances and can be considered for climate and atmospheric research.

In this paper an overview is given of some of the problems to be faced and the approaches to their possible solutions, in the design of a Doppler wind lidar. The emphasis is devoted to the causes of degradation of the heterodyne efficiency and to the line purity requirements for the local oscillator. Both the above aspects directly influence the signal to noise ratio and, as a consequence, the performance of the system. In particular, the first aspect refers to the number of speckles, to the alignment required between signal and local oscillator and to the beams matching characteristics in terms of wavefront and polarization; the second refers to the relative intensity and phase noise of the local oscillator and to the constraints in terms of its power level and in the position of the intermediate frequency stage. Although the receiver design characteristics are responsible for the system sensitivity, in the heterodyne system there are some direct links to the transmitter design specifications like the divergence and the tunability, that must be evaluated for the optimization of the overall system performance. For this reason a few aspects of the transmitter specifications are evidenced, in order to obtain a good trade-off between performance and real feasibility. A few suggestions are given in order to potentially improve the performance of a spaceborne Doppler wind lidar, considering that the technological impact on the system is at the boundary of the feasibility.

The transmitter laser is recognised to be one of the most critical technologies for space-based Doppler windlidar [1].
We present initial evaluation of the performance of an e-beam sustained device in the 1OJ, 10 Hz class. Lifetime issues
are addressed in a subsidiary paper. We describe the design of the device and the results of a number of characterisation studies:
1) General nonoptical tests of gas circulation and heat exchanger efficiency. 2) Performance optimisation to maximise multimode efficiency as a function of energy loading, main discharge
E/N and gas composition, all tests allowed for optimisation of cavity extraction. 3) Characterisation of the novel plasma anode electron gun with respect to beam uniformity, secondary electron
concentration, and current constancy. 4) Optical characterisation to examine operating wavelength, pulse shape, beam profile in the near and far-field,
output energy and electrical to optical conversion efficiency, and frequency behaviour during the pulse.

The European Space Agency program for Development of a CO2 Laser for Spaceborne Doppler Wind Lidar Applications addresses both performance and lifetime aspects. Lifetime issues are of particular importance due to the 109 pulse life requirement for a spaceborne laser operating continuously at 10 Hz for a period of three years. Particularly critical lifetime issues for an e-beam sustained laser have been identified as the electron transmitting metal foil separating the electron gun and the laser, and the gas life. Four areas of study have been undertaken to address the foil and gas lifetime issues: Parametric Study of Gaseous Catalysis to determine the range of operating conditions under which oxidation of CO by high energy electrons can be expected to offset dissociation of CO2, thus eliminating the need for solid catalyst. Extended Sealed Runs to demonstrate long life in a representative laser system of the actual size required. Several runs of 107 pulses, and one run of 6.5 X 107 pulses, have been performed. The Foil Thermal Profile has been monitored using a pyroelectric vidicon camera to determine the maximum temperature reached by different candidate foil materials under representative conditions. High Temperature Foil Fatigue tests of 109 pulses have been carried out to simulate the effect of the laser pressure pulse, by performing fatigue tests on foil materials at high temperature.

The edge technique can provide high accuracy spaceborne wind measurements as well as high spatial resolution, high accuracy ground and airborne measurements. Global wind measurements can be made with the edge technique from space with an accuracy of 1 m/s and a vertical resolution as high as 150 m in the boundary layer and 1 km through the troposphere. The edge technique can also be used for ground and airborne measurements with a spatial resolution and accuracy as high as 15 m and 20 cm/s. We have recently demonstrated this capability and present these measurements in this paper.

One way of obtaining an information on wind velocity in the atmosphere from measurements by Doppler lidar is to estimate the power spectral density of the Doppler signal and its parameters, namely, the center of the spectrum and the width of the spectrum. The first of them is generally taken as the radial wind velocity component and the second one defines the radial velocity variance which contains information on the rate of turbulence energy dissipation. The accuracy of measurement of wind velocity and turbulence depends on the accuracy of estimation of the Doppler spectrum. The paper is dedicated to analysis of estimation of the power spectrum of the Doppler signal and the spectrum parameters. In contrary to previous works in this paper the non-Gaussian statistics of lidar signal due to atmospheric turbulence is taken into account. It is shown, that in consequence of non-Gaussian statistics the relative rms error of spectrum estimation exceeds unit if the measurement duration is far less than the period of correlation of wind velocity fluctuations and the length of sensing volume (Delta) z is far less than outer scale of turbulence LV((Delta) z/LV<<1). By the increase of (Delta) z the relative error can be reduced to the level determined by the number of statistical freedom degrees. The increase of dimension of the sensing volume leads also to decrease of the error of estimating both the center and width of the spectrum. The error of estimation of the width decreases with increasing the ratio (Delta) z/LV monotonously. The error of estimation of the spectrum center decreases up to the level determined by the signal-to-noise ratio and the time of measurement.

A behavior of the measurement error of the average Doppler frequency as a function of parameters of the turbulent atmosphere and technical parameters of the Doppler lidar is considered based on the maximum likelihood method and statistical theory for estimating the random process parameters. It is shown that the measurement error of the average Doppler frequency first decreases and reaches the minimum point, then it increases with an increase of the scattering volume length for a locally-isotropic model of the turbulence corresponding to the `2/3 law'. The value of the scattering volume length which corresponds to the minimum measurement error depends on the energy dissipation rate and technical parameters of the Doppler lidar: the signal-to-noise ratio, time of sample, and number of samples. The results of this paper are in agreement with the results published in Ref. 1 - 7 in limiting cases.

During the last 10 years the DLR container LDA (Laser Doppler Anemometer) was used for many wind related measurements in the atmospheric boundary layer. The experience out of this were used to construct an airborne Doppler lidar ADOLAR. Based on the available Doppler lidars it is now proposed to perform a campaign to demonstrate the concept of the spaceborne sensor ALADIN, and to answer some questions concerning the signal quality from clouds, water and land. For the continuous wave CO2 laser, the energy is focused by the telescope into the region of investigation. Some of the radiation is back scattered by small aerosol particles drifting with the wind speed through the sensing volume. The back scattered radiation is collected by the telescope and detected by coherent technique. With the laser Doppler method one gets the radial wind component. To determine the magnitude and direction of the horizontal wind, some form of scanning in azimuth and elevation is required. To keep the airborne system compact, the transceiver optics is directly coupled to a wedge scanner which provides the conical scan with the axis in Nadir direction from the aircraft. The system ADOLAR was tested in 1994. Results of the flight over the lake Ammersee are presented and are compared with the data of the inertial reference system of the aircraft.

At the Wettzell Laser Ranging System (WLRS, Germany), the Avalanche Photodiode SP114 was operated successfully at a wavelength of 532 nm as well as at 1.064 micrometers . The detector was found to have a high sensitivity, a high accuracy and was convenient to use. To analyze it's sensitivity in detail, a common experiment was planned and carried out, at the Lunar Laser Ranging Station in Grasse (France). This ranging experiment to the most demanding targets allowed a direct comparison between the well known characteristics of the RCA 31034a photomultiplier and the SP114. Under varying configurations, measurements were taken from all lunar targets. It was found, that the SP114 had a sensitivity and accuracy far above the photomultiplier.